The purposes of these investigations are to gain further information on the events occurring during muscular atrophy which is induced by immobilization of the hind limbs of rats. When muscles are fixed at a position which is less than resting length, the loss of weight is exponential as a function of time. The time taken to decrease to one-half of the final decrease in weight observed at the new apparent steady state of atrophy is 4-6 days. On the other hand, when muscles are fixed in a stretched position, muscles undergo a transient hypertrophy prior to atrophy. The duration of the transient hypertrophy seemed to be inversely related to muscular size. The proteins cytochrome c and citrate synthase were lost at a rate faster than the muscular mass during atrophy. In contrast, there was no decrease in myoglobin content in atrophying fast-twitch muscles within limbs fixed by plaster casts.